This invention relates to a method and apparatus for sensing the presence, measuring or counting fish such as salmon in fresh water such as a stream, whereby control or assessment of fish in migration across a selected site may be achieved. In use the apparatus is positioned at a suitable point in the path of migration and a sensor detects the presence of fish which can be recorded in various ways, such as on a numerical counter, a printer or a chart recorder. In this way data on the migration as a function of time can be assessed.
Known methods for counting fish in streams comprise directing the fish into a trap by obstructing the stream except for an opening into the trap and then counting by observation or directing the fish into a tunnel or a rectangular duct or opening and sensing by photocells or by the change in conductivity a fish causes between two electrodes as a result of the higher electrical conductivity in the body of the fish as compared with the water. All these methods require specially constructed surroundings in that the stream is made more narrow and the free passage is then through a counting tube, or alternatively fine meshed nets direct the fish into the sensor area. These methods are expensive in initial cost if permanently installed and when nets are used the operating costs are high. A variation of these methods is to use a fyke-net which is a special trawl like net placed in the stream and which ends in a bag or an open box. The fish are then counted by visual inspection. This method is most suitable for fish migrating downstream, such as smolt.
Acoustic methods are also used with a number of sensors arranged across the path of migration. This equipment is also expensive.
The counting of fish in open streams has however remained a problem and this invention provides a method and an apparatus for this purpose using an electrical current for measuring and counting fish in open streams or water using the higher conductivity of a fish as compared to fresh water and a linear sensor placed across the path of the migrating fish which does not require narrowing of the stream. The invention also provides in a preferred embodiment means for reducing noise and disturbing signals acting on the sensor.